1. Field of the Invention
This invention relates generally to implantable medical devices, and more particularly to methods, apparatus, and systems for dynamically monitoring intermittent lead condition problems associated with an implantable medical device.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated by reference in its entirety in this specification. Electrical stimulation of the vagus nerve may be provided by implanting an electrical device underneath the skin of a patient and performing a detection and electrical stimulation process. Alternatively, the system may operate without a detection system if the patient has been diagnosed with epilepsy, and may periodically apply a series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
Typically, implantable medical devices (IMDs) involving the delivery of electrical pulses to body tissues, such as pacemakers (heart tissue) and vagus nerve stimulators or spinal cord stimulators (nerve tissue), comprise a pulse generator for generating the electrical pulses and a lead assembly coupled at its proximal end to the pulse generator terminals and at its distal end to one or more electrodes in contact with the body tissue to be stimulated.
Occasionally, damage to the lead assembly can occur, which may cause various operational problems. Impedance measurements may be used to assess the integrity of the electrical leads that deliver the stimulation provided by a pulse generator. A change in the impedance across the leads that deliver the electrical pulses may be indicative of either or both of changes in a patient's body or changes in the electrical leads themselves. For example, damage in the lead, which may be induced by a break in one or more filaments in a multifilament lead wire, or changes in the body tissue where stimulation is delivered, may affect the efficacy of the stimulation therapy. Therefore, it is desirable for changes in the lead impedance, which may be indicative of various changes or malfunctions, to be accurately detected.
For instance, the integrity of the leads that deliver stimulation is of interest to insure that the proper therapy dosage is delivered to the patient. Some IMDs, most notably pacemakers, provide a voltage-controlled output that is delivered to one or more body locations (typically the heart). Other IMDs, such as a vagus nerve stimulator device developed by Cyberonics, Inc., provide a current-controlled output. Generally, however, state-of-the-art measurements of lead impedance involve an analysis of the delivery of a voltage signal from a capacitive (C) energy storage component through the resistive (R) lead impedance and an examination of the decay of that signal based upon a time-constant proportional to the product of the resistance and capacitance (RC). The total equivalent impedance present at the leads and the known energy source total equivalent capacitance cause a time-constant discharge curve. As the voltage on the capacitance is discharged through the resistance, the exponential decay of this voltage may be monitored to determine the decay time constant RC. From that time constant and an estimate of the known equivalent capacitance C, the equivalent resistance R presented by the leads may be mathematically estimated. However, this type of measurement may lead to inaccuracies for a number of reasons, including the fact that the discharging of the voltage signal may be affected by other resistances and capacitances in the system, the accuracy of the capacitor, the time, voltage, and algorithmic accuracies of the measurement system, and the like.
The quality and integrity of the electrical signals that are sent via the leads are important in proper delivery of therapy. However, leads may occasionally fail, exhibiting problems, such as an electrical short, or a break in one or more conductors associated with the lead. State of the art implantable medical devices offer integrity tests to diagnose lead failures. Direct or indirect lead impedance measurements may be employed, which may identify dramatic or gross lead failures. However, these state of the art analyses may not detect intermittent failures. Intermittent failures can occur for a variety of reasons and may only occur during certain time periods or during certain physical movements by the patient. Normally, leads may be connected properly to a conductor when the lead is in a “positional range” (i.e., proper position where conductors of the lead are properly electrically connected). Outside this positional range, the conductor associated with the lead may lose electrical connection with the lead. This may result in position-dependent intermittent loss of therapy. Further, this type of problem may be difficult to detect. Designers have attempted to alleviate some of these problems by performing periodic impedance measurements. However, the more the frequent the impedance measurements, the more energy that is consumed, and thereby, essential battery life may be prematurely diminished.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.